Sandberg, did you get the butt rest off? If not, here is an expanded version of Franklin's explanation.
The 400 has two small holes in bottom side of the seat pan/shell, one on each side of the trunk light. In each hole, there is a small phillips head screw. The screws are attached to each end of the tubes that slide into the seat on the butt rest. If I remember correctly, the butt rest must be in the all the way back position, unlike the 650 in which the butt rest is moves forward to expose the screws. Remove the screws. On my 400, one screw was very difficult to remove and I almost stripped the head. After you remove the screws, push the buttrest adjustment lever and pull the butt rest all the way out of the seat. It comes out very easy.
Do not worry about damaging any thing (except stripping the screw heads). The screws only stop the butt rest from completely sliding out of the seat and are not attached to anything other than the butt rest tubes.
If you feel need more leg room, you will most likely find riding more comfortable without the butt rest.
As far as changing the height, I plan doing the same thing. My plan is to only raise the butt rest 2 or 3 inches. The stock butt rest frame extends at an angle and too far foward on the seat to give you all the extra room available on the seat. My riding buddy cut the bottom 1/3 of his butt rest frame off and recovered the frame so that he could get the maximum space back on the seat. However, he did not raise the height because he has a Utopia driver back rest on his 400. I am either going to cut the bottom of the frame off like my friend did and have a steel plate welded to it or I am going to get a machine shop to make me a new butt rest frame and have it recovered. I am leaning toward the later, because it would be earlier and not damage the stock butt rest. The new frame should be very easy make, a steel plate with two tubes welded into it.
If you have any other ideas let me know.